In conjunction with the drastic advance of radio communication technology in the recent years, there is an increasing demand for electronic parts adapted to operate in a frequency band of several hundred megahertz to several gigahertz or higher. Also, in conjunction with the size reduction of radio communication devices as typified by cellular phones, high-frequency electronic parts mounted in such devices are also required to be reduced in size and price. Various integration techniques have been applied to manufacture multilayer ceramic parts.
In multilayer ceramic parts, a ceramic material and a conductor material have been co-fired so that one or more functions are incorporated in one part. Such multilayer ceramic parts are manufactured by stacking a ceramic material and a conductor material by a printing or sheeting technique to form a laminate, dicing the laminate to a desired shape and dimensions and then firing the shaped laminate or firing the laminate and then dicing the fired laminate to a desired shape and dimensions, and thereafter forming external conductors if necessary. Consequently, the multilayer ceramic parts are of the structure having internal conductor layers each between ceramic layers. Silver, copper, etc. were generally used as the internal conductor suitable for high frequency, especially microwave. In the above-mentioned manufacturing process, it was believed that the melting of the internal conductor should be avoided in order to accomplish satisfactory properties. It was thus believed that firing should be done at a temperature not higher than the melting point of the internal conductor. For this reason, it was believed impossible to use silver, copper and similar conductive materials having a low resistivity, but a low melting point as the internal conductor in combination with ceramic materials to be fired at high temperature.
Meanwhile, JP-A 252618/1994 of the present applicant discloses a method for forming a low-melting internal conductor as mentioned above in a ceramic material which is not adapted for low-temperature firing. This method, which is designated conductor melting method, is by firing a multilayer ceramic part at a temperature between the melting point and less than the boiling point of a conductive material used as an internal conductor, whereupon the fired conductor material solidifies in a cooling step to form an internal conductor. This method has the tendency that the grain boundary between metal gretins which is formed as the molten conductor material solidifies becomes thin enough to be regarded extinguished in a substantial sense and the interface between the ceramic material and the internal conductor has less irregularities. The internal conductor is thus reduced in high-frequency resistance and the Q value in the high-frequency region is increased. Further, inexpensive conductive materials having a relatively low melting point such as silver and copper can be used as the internal conductor. Moreover, the method is fully advantageous in productivity and cost because the ceramic material and the internal conductor can be simultaneously fired.